Replaced elaborate install logic with just a bunch of symlinks.

Maybe I'm naive but even after thinking about it for quite
some time I can't see why it should hurt to just build the
cc65 binaries from the sources "in place" and have 'make
install' simply create some symlinks in usr/local/bin.

The new Makfile builds the binaries with builtin search paths
matching their build location. So the symlinks in usr/local/bin
allow to allow to use them out-of-the-box without additional
environment variables.